Looks like Adafruit are in the process of manufacturing their prototype Metro M4 boards: Adafruit Prototyping League tackles the Metro M4 #ManufacturingMonday (video) « Adafruit Industries – Makers, hackers, artists, designers and engineers!.
From their Gibhub SAMD51 repo, it also looks like they're also planning on a Feather M4. I believe that these initial boards will use the SAMD51J19A, 64-pin device with 512K flash and 192K RAM.
Adafruit's code now allows you to seamlessly upload code to the SAMD51 via the native USB port using the Arduino IDE. I've now ported the firmware from my SAMD21, Arduino Zero based flight controller over to the SAMD51 and have successfully flown the new microcontroller in a tricopter frame. I'm using the SAMD51J20A, 64-pin device with 1M flash and 256K RAM.
The SAMD51 is similar to the SAMD21 pinout in terms of power and port pins, except for a three way swap of the VDDCORE, PA27 and VSW pins, used for microcontroller's internal regulated supply. Like the SAMD21 it also uses the same 32.768kHz crystal.
Internally the SAMD51 uses a digital frequency locked loop to generate a 48MHz clock source, this is then divided down and ramped up to 100MHz and 120MHz using two fractional digital phase locked loops. All these clocks are available as sources for the microcontroller's internal peripherals. Having the 48MHz clock source is nice, as it allows peripherals to be clocked at the same rate as the SAMD21. The 100MHz clock is necessary, as this is the maximum clocking frequency of some of the internal peripherals such as the SERCOM modules. The 120MHz clock drives the processor core.
From an Arduino point of view the code runs exactly the same as the SAMD21, just around 2.5 times faster. If your sketch uses a lot of single precision floating point calculations, it'll run faster still, on account of the microcontroller's hardware FPU (Floating Point Unit).
For those using register manipulation, some of the peripherals like the SERCOM remain almost identical to the SAMD21, while others remain strangely familiar, but different. The TC timers for example are now more akin to the TCC timers in terms of operation.
The SAMD51 isn't just a turbo-charged version of the SAMD21 though. It has more memory, dual ADC and DAC, multiple interrupt handlers per peripheral, more timers and timer channels, plus a number of other specialised on-board peripherals not available on the SAMD21. The only main disadvantage that I've found with respect to the SAMD21, is the lack of flexibility with regard to multiplexing of the SERCOM's transmit pin, but this is only really a circuit board layout issue.
The other nice thing about the SAMD51 and SAME51 families is that they're available 48, 64, 100 and 128-pin packages. This makes it possible to produces various sizes of board, while leveraging a single SAMD code base. In this regard, Adafruit have also mentioned about making a Mega sized board with the SAMx51 100-pin variant.